The loss of motor control due central nervous system (CNS) trauma can be both a psychologically and physically devastating injury. A number of recent studies have reported the feasibility of using electrical stimulation to help patients walk following spinal cord injury (SCI). The use of electrical stimulation offers many advantages over traditionally used orthotic supports such as long leg braces. The use of electrical stimulation in individuals who have sustained SCIs has been shown to be safe and produce significant physiological benefits. One of the limiting factors in the present application of electrical stimulation to help individuals who lack adequate CNS control of movement is the rapid skeletal muscle fatigue which accompanies its use. The overall objective of this project is to identify stimulation patterns which minimize fatigue by most efficiently activating skeletal muscle. All published studies which have used electrical stimulation to help individuals with damaged central nervous systems to regain function have used constant frequency trains (ie, all pulses within a train are separated by regular intervals). In contrast, recent work has suggested that varying the stimulation frequency within brief trains of activation will produce greater forces from skeletal muscle as the muscle undergoes fatigue. Animal and healthy human subjects will be used to characterize the response of skeletal muscle to variable frequency trains. Four sets of conditions will be tested: isometric contractions of non-fatigued and fatigued muscles and non-isometric contractions (concentric and eccentric isokinetic contractions) of non-fatigued and fatigued muscles. A model will be developed which will use the contractile characteristics of each muscle to predict the force output of the muscle in response to any pattern of stimulation. the observed and modeled data will then be used to identify the "optimal" variable frequency train for each condition. The effectiveness of the identified "optimal" variable frequency train under each conditions in reducing fatigue will be compared to the traditional, constant-frequency trains. As technology continues to improve, the increased use of electrical stimulation to assist in regaining lost CNS function appears inevitable. Only through optimization of all relevant parameters can the true potential of this modality be realized. This study proposes to optimize an important parameter, the stimulation frequency.